Nearly perfect
I have mouse shoulder problem so I figured a trackball mouse would help. In general, the trackball mouse is good. It's solid: the ball is big and the pointer movement is precise. The buttons is large enough that you can click without having to look.
However, the biggest problem is the steep incline of the device. Even with the included wrist rest (which is stiff and doest not fit to the device properly), my wrist is always at an upward angle when using this mouse, causing wrist discomfort on my left hand which is not my primary mouse hand within one day. However the discomfort went away the next day. Note that I placed the mouse alongside my keyboard at slightly below my elbows, on my keyboard tray, which is the usual place.
For better experience, download and use TrackbalWorks to configure both mouse pointer speed and acceleration to fast. The acceleration setting is quite important as it helps with the precision of mouse pointer control. If you want more precision, slow down the mouse and you get more control. When you want to move the pointer fast, just whip the ball and the acceleration speeds up the pointer movement.
The wrist rest feels cheap even though is kinda work.
One of the best features of this mouse is, however, the scrollwheel. I know some people think it's noisy and grinding. But to me, it works great. I can scroll without changing hand position. It helps with productivity.
Update (May, 14th, 2017):
Having using it for several months, I feel another followup is justified.
The trackball still works great. The scrollwheel spins without hiccup. My wrist doesn't hurt anymore, seems it adjusts to the inclination.
One month after buying the trackball, I accidentally dropped the ball while cleaning and it got a small nick. The trackball still worked but it jumped a bit when the nick scrolled over the jewel. I contacted Kensington customer service, asking if buying a replacement ball was possible. After a few days, Joyce Mah replied back, asking for my purchase information so he could send a free replacement ball. I told him I didn't have the receipt with me anymore and was willing to pay for the ball. To my surprise, he asked if I could send him the picture of the damaged ball instead. I agreed. After a few days, he said the customer service sent me the replacement ball. A week passed, a package from Kensington arrived and, lo, TWO replacement balls were in the package. I was quite happy. The replacement balls work great.
Verified purchase: YesCondition: New